The present invention relates to communications systems, and more particularly to broadband communication systems.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
Broadband wireless communications requires the use of a segment of the electromagnetic spectrum. One detriment to successful data communication is the presence of other transmissions in the utilized spectrum. For example, the MMDS band, including 2.5-2.7 GHz, is envisioned as a prime candidate for the expansion of broadband wireless Internet access services. However, wireless data transmissions there may be subject to interference from incumbent narrowband analog video transmissions. Another band slated for broadband wireless Internet access is the U-NII (Unlicensed National Information Infrastructure) band at 5.725-5.85 GHz, but the unlicensed Internet access transmissions there are subject to interference from other such transmissions as well as other services entitled to access to the band. Aside from spectrally overlapping unwanted signals, there is also self-interference resulting from signal reflections between the transmitter and receiver.
One approach to mitigating the problem presented by interference is to increase the use of forward error correction (FEC). In forward error correction, error correction coding is used to incorporate redundancy in the transmitted information so that the receiver is able to detect and correct errors in the face of channel impairments such as noise and/or interference. The more redundancy that is added, the greater the level of interference that the receiver may tolerate without impairment of communications. The disadvantage of this approach is that link capacity used to incorporate redundancy reduces the payload data carrying capacity. Furthermore, the redundant data is typically spread across the entire channel even though the interference may be limited to a rather narrow segment.
Another approach to handling interference is to incorporate knowledge of the interference environment in the planning of the network. Before deploying the broadband wireless communication network, a site survey is carried out to measure the interference environment in which the network will be expected to operate. Relevant potential interference that could adversely affect the link performance is identified. Where appropriate, transmitter power is increased, larger more direct antennas are employed, or additional base stations are deployed.
This approach that relies on work done at the network planning stage carries multiple drawbacks. The network modifications adopted in response to the measured interference increase deployment costs and/or reduce network capacity. Furthermore, the long-term characteristics of the channel cannot be predicted based on measurements made at the time of deployment. Interference may change frequency and location, even in real-time, rendering modifications made at deployment time essentially useless.
What is needed are systems and methods for combating interference that minimize loss of data carrying capacity and can handle rapid variation in the interference environment while adding minimal cost to network deployment.